Here's
a quick way to cut a dado on a table saw. It only requires a combination saw blade
and two spacers. The spacers are used to establish the two sides of the dado.
Then the material between the sides is wasted out.

Spacer Thickness  The key to making this work is the thickness of
the spacers. the first spacer mathces the thickness of the workpiece that fits
into the dado. (A scrap piece from the project works just fine.) And the second
spacer equals the thickness of the blade. (I use a piece of 1/8" hardboard with
a single strip of masking tape.)

First Side  To
cut the first side of the dado, start by clamping one spacer (the scrap from the
project) to the rip fence, as you can see in the drawing at right. Then position
the fence so the saw blade aligns with the near side of the dado (see detail 'a').
After locking the fence in place, butt the workpiece against the spacer and use
the miter gauge to push it through the blade. Note: You can use this same technique
for cutting grooves. The only thing to keep in mind is that the spacers need to
be the same length as the fence.

Second Side  The second side of the dado is cut with the rip fence
in the same exact position. Only this time, you'll need to replace the first spacer
with one that matches the thickness of the blade, as shown in the photo above.
With this spacer clamped in place, it's just a matter of making a scond pass (see
detail 'b').

Remove Waste  All that's left to complete the dado is to remove the
rest of the waste. To do this, just "nibble" away the waste by making as many
passes as needed (see detail 'b').

Ridges  One thing
you'll notice about a combination blade is it leaves ridges on the bottom of the
dado. If the end of the dado is going to be covered up, you can just leave the
ridges. But if it's exposed, you may want to sand the bottom of the dado flat.
For a handy way to do this, take a look at the drawing at left.